The invention herein disclosed and claimed pertains to the field of data transmission. More particularly, the invention is in the field of modulation techniques for data transmission, which includes the field of minimum-shift-keying (MSK). The field of the invention includes the field of MSK modulators constructed using digital components.
When applied to the transmission of digital information, the technique of MSK modulation involves the use of a pair of related transmission frequencies between which a modulator is switched according to the state of the data to be transmitted. If the two frequencies are denoted f.sub.u and f.sub.1, where f.sub.u denotes the greater and f.sub.1 the lesser of the two frequencies, then f.sub.u will be transmitted in the case of a current digital bit which follows an immediately preceding digital bit of the opposite value. Similarly, the transmission of f.sub.1 will denote the presence of a digital bit following an immediately preceding digital bit of the same value. The values of the frequencies are so selected that a change from one to the other will result in no change of phase in the composite modulated waveform. Reference is given to the article "Simplified MSK Signaling Technique," by Amoroso and Kivett in the IEEE Transactions on Communications, April 1977, pp. 433-441.
As far as is known, the application of MSK modulation theory to modulator design involves the use of analog circuits to develop a phase-continuous, modulated wave by means of successive mixings of sinusoids under the control of a phasor differential signal derived from the stream of data to be transmitted. See, for example, "The Effect of Tandem Band and Amplitude Limiting on the E.sub.b /N.sub.o Performance of Minimum (Frequency) Shift Keying," by Mathwich, et al., in the IEEE Transactions on Communications, October 1974, pp. 1525-1539. Because of the use of analog circuitry, the precise phase relationships required for analog MSK modulation are difficult to maintain in the face of changes in ambient temperature and of component ageing. Both effects can vary the value of components which generate and process the sinusoids; variations on component values are reflected in unwanted shifts in frequency and phase of the waveforms.
The employment of digital circuitry can provide an MSK modulator with the capability of maintaining phase precision in the face of ambient temperature change and component ageing because of the greater inherent operational stability enjoyed by digital devices. In addition, programmable digital devices provide the opportunity to maintain a portfolio of automatically selectable data transmission rates which can be changed by simple reprogramming as conditions require. A programmable transmission rate portfolio can include preselected frequency values which can be synthesized quite simply by a change in the frequency of a digital clock source. Needs for such complicated analog circuits as mixers, balanced modulators, and precise amplitude combiners can be eliminated.